the least you could do is put the sub against one wall; that would give you some gain compared to a free field position.
also, there are many subs that will put out good quality sound and good loudness at 25Hz or so; the main difference is that the volume on all speakers will be lower than in an indoor installation, and you will also get much less standing waves.
if you're looking for realistic sound levels, then you need loud, specially for the sub, remembering that the max. levels for movies is 105 dB SPL for the main speakers and 115 dB SPL for the sub.
you'll be lucky to get that outside unless you sit close to the speakers.
Last edited by J. B.; 06/21/11 06:01 PM.
